On-board chargers of electric vehicles (EVs) are generally equipped with AC-DC converters and DC-DC converters. The AC-DC converter converts commercial AC power to DC power, and the DC-DC converter may charge a battery in a constant-current/constant-voltage charging manner. In general, fuel efficiency is an important factor in evaluating the performance of an electric vehicle, so it is important to implement a charger that is highly efficient and compact.
For this purpose, a phase-shifting full bridge converter is most commonly used as the DC-DC converter for an on-board charger because it may achieve a zero voltage turn-off condition of primary switch elements. However, the phase-shifting full-bridge converter has problems such as a circulation current-related conduction loss, a duty cycle loss, a narrow zero voltage switching range of switch elements of a lagging leg, high voltage oscillation and reverse recovery of rectifying diodes.
Accordingly, various methods for overcoming the problems of the phase-shifting full bridge converter have been studied.
For example, a method of adding various types of auxiliary circuits to a primary circuit to extend the zero voltage switching range of switch elements has been proposed. However, this method has problems of increasing the duty cycle loss, causing an additional cost and decreasing efficiency since a large external inductor is added.
Alternatively, a method of adding a passive lossless clamp circuit to a secondary circuit to mitigate the voltage ringing of rectifying diodes and to solve problems arising from the circulating current has been proposed. A representative example of the clamp circuit is a capacitor-diode-diode (CDD) circuit in which a clamping capacitor is included in the resonance or non-resonance of a leakage inductor of the transformer. In this case, the primary current of the transformer may be reset during a freewheeling period. However, the switch elements provided at a leading leg of the primary side have a problem of being turned on under the hard switching condition.
Alternatively, a method of adding an active clamp circuit to a secondary circuit so as to achieve a zero-voltage/zero-current switching condition of primary switch elements has been proposed. In this case, by properly controlling the clamp switch, it is possible to turn on the zero voltage switching of the switch elements provided at the leading leg and turn off the zero current switching of the switch elements provided at the lagging leg. However, there is a problem that soft switching of the clamp switch is impossible.
Alternatively, a method of adding an active energy recovery clamp and an auxiliary circuit for suppressing the circulating current to a secondary circuit has been proposed. However, this method requires a large number of additional elements, which gives complexity in implementing, thereby reducing the stability and efficiency of the converter.
In addition, a phase-shifting full-bridge converter including a voltage-doubler-type rectifier has been proposed as a method using rectification between a leakage inductor and a secondary rectifying capacitor for energy transfer. However, this method also has problems in that the current stress of the rectifying diodes is large, the circulating current is incompletely removed, and the condition for achieving zero current switching of the primary switch elements is dependent on the load current. Thus, this method is mainly applied to high-voltage and low-current devices.